In the field of power supply technology, synchronous rectification circuits are widely used.
A switch power circuit of the traditional synchronous rectification circuit commonly uses a diode to implement the rectification; however, higher conduction loss of the diode leads to lower working efficiency of a switch power supply of the rectification circuit using the diode. Later, people gradually use a Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET or MOS transistor) or other switch tubes to replace traditional diode to implement the synchronous rectification. FIG. 1 is a circuit diagram of the switch power circuit of the synchronous rectification circuit in the prior art. The switch tubes Q1 to Q4 are the switch tubes of a primary side of the switch power circuit of the synchronous rectification circuit. VIN+ and VIN− are input terminals of the primary side of the switch power circuit of the synchronous rectification circuit. The switch tubes Q5 to Q8 are the switch tubes of a secondary side of the switch power circuit of the synchronous rectification circuit. The switch tubes may be implemented in many forms, for example, through diode, transistor, or MOS transistor. In FIG. 1, all switch tubes Q1 to Q8 using the MOS transistor are taken as an example in the description. Referring to FIG. 1, VOUT+ and VOUT− are output terminals of the secondary side. T1 is an isolation transformer of the primary side and the secondary side of the switch power circuit of the synchronous rectification circuit, and a first capacitor C1 is an input filter capacitor of the primary side of the switch power circuit of the synchronous rectification circuit. As shown in FIG. 1, the switch tubes Q5-Q8 of the secondary side of the switch power circuit of the synchronous rectification circuit use the MOS transistors to replace the diodes in the prior art, so as to greatly reduce conduction loss and improve efficiency of the power supply. But after the MOS transistors are conducted, the current flows bidirectionally. When an input power supply produces a low-impedance fall, the synchronous rectification technology will produce a great output reverse current surge, which may easily damage the MOS transistors. Therefore, the switch power supply of the synchronous rectification circuit using the MOS transistors has a potential risk in terms of the reliability.
In order to enhance the reliability of the switch power supply of the synchronous rectification circuit, in the prior art, an anti-reverse diode or the MOS transistor is added at a voltage input terminal of a switch tube rectification unit of the primary side, so as to quickly switch off the reverse current when an conduction current is reverse, thereby effectively protecting the switch power supply of the synchronous rectification circuit.
In the implementation of the present disclosure, the inventor finds that the prior art at least has the following problems. In the prior art, the anti-reverse diode or the MOS transistor is added to the voltage input terminal of the switch tube rectification unit of the primary side, which can prevent a reverse current surge produced by the power supply to the input terminal when the input power supply produces the low-impedance fall to some extent. However, the anti-reverse diode or the MOS transistor increases the system cost. Moreover, since the anti-reverse diode or the MOS transistor is configured in a main circuit of the switch power circuit of the synchronous rectification circuit, power consumption of the circuit is increased, and working efficiency of the power supply is reduced.